As shown in Patent Document 1, crawler devices for carrying load, having features for ascending and descending stairs are known in the art. The crawler device of the Patent Document 1 includes a device body having a base and a pair of main crawlers mounted on the left and right of the base and a pair of auxiliary crawlers respectively rotatably connected to one end portions of the main crawlers.
In the crawler device mentioned above, the length of the main crawler is greater than twice the distance between adjacent nosings of the stairs, which enables the crawler device to ascend the stairs stably with the main crawler straddling over three stair nosings. The length of the auxiliary crawler is half or smaller than the length of the main crawler.
When the crawler device ascends stairs, the auxiliary crawler is positioned forward in a direction of travel, being raised at a predetermined angle. When the auxiliary crawler hits the nosing of the first stair, the auxiliary crawler starts ascending the nosing of the first stair. Subsequently, an end portion of the main crawler reaches the nosing of the first stair, and the main crawler proceeds to ascend nosings of the second and the third stairs. After that, the main crawler ascends the stairs maintaining the same tilt angle as an inclination of the stairs.
The device body has a tilt sensor. When a measured tilt angle of the device body reaches a predetermined angle, the auxiliary crawler is rotated downward to be located on a line extending longitudinally from the main crawler, i.e. to be in alignment with the main crawler.
In the crawler device mentioned above, a diameter of a distal end portion (end portion farther from the main crawler) of the auxiliary crawler is smaller than that of the main crawler. Therefore, when the crawler device ascends stairs with the auxiliary crawler in alignment with the main crawler, the auxiliary crawler never contacts nosings of the stairs. Only the main crawler is used to ascend the stairs.    Patent Document 1: Japanese Patent Publication No. 3796490